Delayed coking of oil residues

ABSTRACT

This invention relates to petroleum processing, in particular to producing coke with a delayed coking process and to an assembly for trapping the products formed during the coke steaming and cooling processes. 
     Petroleum coke deposits in delayed coking reaction drums. When one drum fills up, the flow of feed into that drum is switched off. The temperature in the switched off drum rises to 420-450° C. To remove coke from the drum, the drum needs to be water-cooled to 60-90° C., but the petroleum vapour products must be removed first and steamed water steam. Low-quality fractionating of the products formed during the steaming and cooling of coke results in losses of petroleum products and in atmospheric pollution.

This invention relates to petroleum processing, in particular toproducing coke with a delayed coking process and to an assembly fortrapping the products formed during the coke steaming and coolingprocesses.

Petroleum coke deposits in delayed coking reaction drums. When one drumfills up, the flow of feed into that drum is switched off. Thetemperature in the switched off drum rises to 420-450° C. To remove cokefrom the drum, the drum needs to be water-cooled to 60-90° C., but thepetroleum vapour products must be removed first and steamed water steam.Low-quality fractionating of the products formed during the steaming andcooling of coke results in losses of petroleum products and inatmospheric pollution.

Table 1 shows a typical composition of the steaming and cooling productsof coke produced by delayed-coking.

TABLE 1 Steaming and cooling products Flow rate, kg/hour 1. Gaseousproducts (C₁-C₄) 300-1000 2. Petroleum products, boiling 1,000-3,000 temperature = 40-350° C., density 650- 950 kg/cm³ 3. Petroleum products,boiling  500-1,000 temperature >350° C., density >950 kg/cm³ 4. Watersteam 5,000-50,000

In existing delayed-coking units, gaseous products are usuallytransferred—as fuel—to a special burner of technological furnaces.Petroleum products with density under 950 kg/cm³ separate easily fromwater by sedimentation and re-enter the technological process. Trappingpetroleum products with density >950 kg/cm³ is extremely difficultbecause their density is close or equals that of water. They cannot beseparated from water by settling, even using de-emulsifiers.

There exists a method for delayed coking of petroleum residue,consisting of fractionating in an evapouration column the primary cokingfeedstock mixed with a recirculater into light fractions and heavyresidue. The residue undergoes a delayed coking process. The vapourcoking products formed in the coking drum undergo fractionating in therectification column into light fractions and bottoms, a part of whichis used as a recirculater (Russian Federation Patent No. 2209826, ClassC10B 55/00, published in 2003).

The drawback of this method is using bottoms gasoil from the bottom ofthe rectification column Bottoms gasoil forms as a result ofcondensation of high-boiling temperature fractions of the cokingdistillate in the bottom section of the rectification column, which areintroduced into the rectification column from the coking drums. Thecoking distillate may contain particles of coke, which get into thefurnace with secondary feedstock. Therefore it can coke up the furnaceand increase the frequency of required overhauls for the entire unit.

Another drawback is large losses of petroleum products and pollution ofthe atmosphere due to the lack of devices for trapping the products ofcoke steaming and cooling.

The delayed coking method most similar to the present invention is theone that consists of heating the primary feedstock in a pipe still,mixing it with a recirculater, producing the secondary feed byfractionating of the light fractions in an evaporator, heating thesecondary feedstock produced in a reaction pipe still, coking it incoking drums, which produces coke and distillates, fractionating thelight fractions produced in the evaporator, mixed with coking distillateproducts, in a rectification column into vapor products, light and heavygas oils and bottoms. The primary feedstock is heated at 400° C., whilewater condensate is introduced into the inlet part of the coil of thepipe still to heat the feedstock. (Patent RU No. 2410409, MPK C01B55/10,published on 27 Jan. 2011.)

The drawback of this method is the insufficient yield of coke products:hydrocarbon gas, benzene, light gas oil and bottoms. There is room forincreasing the yield of that unit.

The new method improves the yield of the unit, while simultaneouslyimproving the yield of the products of coking.

To achieve this goals, the new delayed coking method of petroleumresidue, which includes the coking of primary feedstock, during whichcoke deposits in the drum, fractionating of the distillate cokingproducts into vapour products, light and heavy gas oils and heavybottoms, steaming the coke with water steam, cooling it with water,feeding the products of steaming and cooling into the absorber equippedwith mass-exchange devices, fractionating the steaming and coolingproducts into vapour and liquid phases in the absorber, absorbing thelow-volatility petroleum products from the vapour phase by feeding theresidue from the bottom section of the absorber into a mass-exchangedevice, cooling and condensing vapour components in acondenser/refrigerator, and fractionating the products of cooling into agas, petroleum products, and water. In this invention, the heavy cokinggas oil produced is split into several flows, one of which is used as arecirculant and is mixed with the feedstock in the evapouration drumprior to coking, while the other flow is used to dilute the steaming andcooling products prior to feeding them into the absorber. The bottomsare returned from the bottom section of the absorber to themass-exchange assembly located in the middle part of the absorber (tothe third or fourth mass-exchange plate preferably), while the rest ofthe bottoms is returned into the bottom section of the rectificationcolumn.

A wash agent, heavy gasoil produced by coking, is also fed into thecondenser/refrigerator.

The figure shows a flow diagram of the unit. This flow diagramillustrates the suggested delayed coking method of petroleum residue.

The unit includes heat exchangers 1 for heating the primary feedstock,evapouration column 2 for producing secondary feedstock 3 by mixingprimary feedstock with recirculant 4 (heavy coking gas oil),heating-reaction oven 5 for heating secondary feedstock, secondaryfeedstock coking drum 6, rectification column 7 for the fractionating ofcoking distillate products 8 into heavy bottoms 9 and vapour products10, condenser/refrigerator 11 for cooling vapour products 10,fractionator 12 for the fractionating of the above-mentioned vapourproducts into a gas, light petroleum products and water, absorber 13,equipped with an assembly for mass exchange, such as valve plates forexample, for the absorption of petrol products from the products 14separated in the process of steaming and then cooling of the cokeproduced in the coke coking drum, the vapour phase 15 and the bottoms16, condenser/refrigerator 17 for cooling and condensing of the vapourphase separated in the absorber, fractionator 18 where the condensedproducts are fractioned into gas, light petroleum products, and water,pump 19 for removing the bottoms 16 from the absorber, refrigerator 20for cooling the bottoms, pipelines: 21 for returning the bottoms as anabsorber to the mass exchange assembly and 22 for feeding the bottomsinto the rectification column respectively.

It was experimentally established that mass exchange between the risingvapours and absorbent in the absorber takes place when the number ofvalve plates equals 10.

Rectification column 7 is equipped in its middle part with a system ofpipelines for the removal of light gas oil 23 as a final product andheavy gasoil 24, one flow of which is used as recirculant 4, while theother flow is used to dilute products 14 formed during the steaming andcooling of the coking coke formed in the drum, at the inlet of theabsorber where it is introduced via pipeline 25, or it is fed as anabsorbent into the top mass-exchanger via pipeline 26, or as the washagent 27 into condenser/refrigerator 17. Pipeline 28 is used forremoving gas oil 24 as a final product.

Light petroleum products are returned from fractionator 18 to therectification column via pipeline 29. Pipelines 30 and 31 are used forsteaming the coke formed in the coking drum and for cooling it withwater, respectively.

The method operates in the following fashion.

Primary feedstock for coking is heated in heat-exchanger 1 by the heatof the leaving flows, then directed into evaporator column 2, where itis mixed with recirculant 4 in the form of heavy gas oil, producingsecondary feedstock 3. The secondary feedstock is heated inheating/reaction oven 5, then transferred to coking drum 6 where theforming coke deposits. Coking distillate products 8 are transferred torectification column 7 for fractionating. Gaseous products 10,consisting of a gas, benzene and water steam, leave from the top ofcolumn 7, are cooled in the condenser/refrigerator 11 and fractioned infractionator 12 into a gas, light petroleum products and water. Gasoils—light 23 and heavy 24—are removed from the middle section of therectification column Light gas oil is removed as a final product, whileheavy gas oil is split into several flows. One part of it is used asrecirculate 4, another part is used as diluent 25 of coke steaming andcooling products 14, while the third part is used as absorbent 26, whichis conducted into the upper mass exchanger of absorber 13. Yet anotherpart of heavy gas oil is used as wash product 27 incondenser/refrigerator 17. For this purpose one of the sections of thecondenser/refrigerator is taken off the flows of vapours from theabsorber, so that hot heavy gas oil can be pumped through it during thattime. All other sections continue their normal operation. When one ofthe sections has been washed, the next section is switched over towashing. The remaining part of the heavy gas oil is removed from theunit. Bottoms 9 are removed from the bottom of the rectification columnIt is either mixed with the heavy gasoil removed from the unit or usedas a boiler fuel.

When drum 6 is filled with coke, it is steamed with steam introducedthrough pipeline 30 and cooled with water introduced through pipeline31. Steaming and cooling products 14 are conducted into absorber 13,equipped with 10 valve-plates. The high viscosity steaming and coolingproducts are diluted at the inlet of the absorber with heavy gas oil 25.The vapour steaming and cooling products rise into the top part of theabsorber, where petroleum products are absorbed on the plates by bottoms21 transferred from the bottom of the absorber to the fourth plate fromthe top, and by heavy gasoil 26, introduced onto the top plate of theabsorber.

The liquid phase of the steaming and cooling products flows down intothe bottom part of the absorber, from where it is removed with pump 19and, via refrigerator 20, is transferred in the form of two flows: flow21, as an absorber, is introduced onto the fourth plate of the absorber,while the other flow is conducted into rectification column 7.

Vapour phase 15, consisting of steam, hydrocarbon gases and lightpetroleum products, enters condenser/refrigerator 17, from where it issent to fractionator 18. The gases, light petroleum products and waterare separated. The gases go to the flare system, the light petroleumproducts 29, mixed with the bottoms from the absorber, are returned intothe lower part of rectification column 7, while water is sent topurification.

Diluting coke steaming and cooling products with heavy gasoil prior totheir introduction into the absorber reduces concentration oflow-volatility bottoms, which are removed from the bottom of theabsorber and is introduced as an absorbent. This suggests that thequality of fractionating has improved.

When heavy gas oil is introduced onto the top plate, low-volatilitycomponents are diluted and washed off the plates located above theplates to which the absorbent has been introduced from the bottoms partof the absorber. This reduces significantly the quantity oflow-volatility components carried out through the top of the absorber.

Introducing absorbent to a plate in the middle part of the absorber (the3^(rd)-4^(th) one from the top, preferably) improves absorption andreduces the quantity of low-volatility components carried out throughthe top of the absorber. This suggests that the quality of thefractionating of trapped products has improved.

Having the condenser/refrigerator continuously washed with heavy gas oilimproves heat exchange. Having a washing sequence of this kind lets thecondenser/refrigerator operate for a very long time without requiring tohave it closed down for cleaning.

Returning the bottoms from the absorber and the separated petroleumproducts from the fractionator into the rectification column resultsbetter trapping of products, while preliminarily mixing the feedstockwith heavy gas oil in the evaporator column, which produces secondaryfeedstock, keeps the feeding of the feedstock heating oven at a constantrate, while keeping up the high yield of this delayed coking method withan assembly for trapping coke steaming and cooling products. (incomparison, in the method that is our prototype, returning the bottomsfrom the absorber and the separated petroleum products from thefractionator into the rectification column resulted in keeping thefeeding rate of the furnace constant, the yield of the method withrespect to the primary feedstock would have to come down by the mass ofthe trapped products).

EXAMPLES SUPPORTING THIS EXPLANATION Example 1 Using the SuggestedMethod

Primary feedstock (West-Siberian tar) was introduced into the evaporatorcolumn, where it is mixed with a recirculant to produce secondaryfeedstock. Secondary feedstock was heated in the oven and transferredinto the coking drum, where coke was produced and deposited. Thepetroleum products produced in the coking process and steam were removedfrom the top of the drum and transferred to the rectification column tobe fractioned into distillate, light and heavy gas oils and bottoms.Heavy gas oil served as a recirculant.

When coke filled up the coking drum, it was steamed with water steam andwater-cooled.

The products of coke steaming and cooling , which include water,gas-phase products, petroleum products boiling at 40-350° C. and withdensity of 650-950 kg/m³, and petroleum products boiling above 350° C.and with density of above 950 kg/ m³ are transferred from the cokingdrums to the absorber. As they are entering the absorber, they are mixedwith heavy coking gas oil with boiling temperature of 250° C. The vapourphase rises into the top part of the absorber and contacts with theabsorbent in the mass-exchange devices. The bottoms from the bottom partof the absorber are used as an absorbent. The bottoms are introducedonto the 4^(th) valve plate from the top. Heavy gas oil is introducedonto the top plate of the absorber as a reflux. Its temperature is120-150° C. The vapour phase is transferred to thecondenser/refrigerator, cooled and moved to the fractionator forfractionating. Heavy coking gas oil of 200-250° C. is introduced as awash liquid into one of the sections of the condenser/refrigerator,taken off the technological process. Gaseous products are removedthrough the top of the fractionator, while water and the petroleumproduct are removed from the bottom. The excess quantity of bottoms inthe bottom part of the absorber is returned into the rectificationcolumn. The water flushed into the canalisation system.

Example 2 Using the Prototype Method

The prototype coking method was used on the same feedstock forcomparison.

Table 2 shows the material balance of the process and technologicalconditions of coking, using the method suggested here in comparison withthe prototype method.

As can be seen from the data in Table 2, the new method of delayedcoking of petroleum residues improves the performance of the unit as awhole, while simultaneously increasing the yield of the coking products:carbon dioxide, benzene, light gas oil and bottoms gas oil.

TABLE 2 Technological Conditions and Material Balance of Coking ExamplesU.S. Pat. No. New 2,410,409 Characteristics method method Technologicalconditions 1. Unit performance in relation to primary 150 150 feedstock,T/h 2. Quantity of recirculate (heavy gasoil), T/h 12 12 3. Unitperformance in relation to secondary 162 162 feedstock, T/h 4. Feedstocktemperature after heating in heat- 250 250 exchanger, ° C. 5. Feedstocktemperature when entering the — 250 furnace, ° C. 6. Consumption ofwater condensate at the inlet — 1.0 of furnace coil, T/h 7. Primaryfeedstock temperature at furnace — 400 outlet, ° C. 8. Secondaryfeedstock temperature at furnace 500 500 outlet, ° C. Material Balanceof Coking, T/h Consumed: primary feedstock 150 150 trapped products ofcoke steaming and cooling 5 5 Produced: carbon dioxide calculated on C₅basis benzene (C₅ fraction: 180° C.) 13 12 light gas oil (180-350° C.fraction) 16.5 15 heavy gas oil (350° C. - coking coke) 46.5 45 bottomsgas oil 28.0 28.4 coke 7.0 5.0 losses 43.5 43.5 Total involved in cokingprocess, T/h 0.5 1.1 150 + 5 150

1-2. (canceled)
 3. A method of delayed coking of petroleum residues, comprising the steps of: coking a feedstock, wherein coke is deposited in a coke drum; fractionating any distillate products in a rectification column into vapour products including light and heavy gas oils and heavy bottoms; steaming coke with a water steam; water cooling the coke; introducing the products of the coke steaming and cooling steps into an absorber equipped with a plurality of mass-exchange assemblies; fractionating the steaming and cooling products into vapour and liquid phases in the absorber; producing low-volatility petroleum products from the vapour phase by introducing the heavy bottoms from a bottom part of the absorber into the mass-exchange assembly; cooling and condensation of the vapour components in a condenser/refrigerator and fractionating the cooling products in a fractionator into gas, petroleum products and water; wherein the produced heavy gas oil is split into a plurality of flows, wherein a first flow is used as a recirculate and is mixed with the feedstock in the evaporator column prior to coking, wherein a second flow dilutes the products of coke steaming and cooling prior to introducing it into the absorber, wherein a third flow is introduced into a top mass-exchange assembly of the absorber, wherein the heavy bottoms are returned from the bottom part of the absorber into the mass-exchange assembly located in a middle part of the absorber, into a third or fourth mass-exchange assembly, wherein a left-over part of the heavy bottoms from the absorber, as well as the separated petroleum products from the fractionator, are returned into the bottom part of the rectification column.
 4. The method according to claim 3, further comprising feeding a wash product including heavy coking gas oil into the condenser/refrigerator. 